Energy converter



Sept. 17, 1968 R CHAPMAN ET AL 3,402,074

I ENERGY CONVERTER v Filed March 25, 1964 2 Sheets-Sheet 1 HEAT EJECTEDHEAT ENCLOSURE COLLECTOR FERM! LEVEL o EMITTER i FERMI LEVEL D|sTANcE.

EMITTER AT COLLECTOR AT TEMP. TE TEMRTC C COLLECTOR v FERMI LEVEL IEMITTER i FERMI LEVEL EMITTERAT COLLECTOR AT TEMP. TE TEMP. T IE 4'?POINT OF MAXIMUM Sept; 17,

Filed March 23, 1964 T ENERGY f X RELATi E ENERGY R. A. CHAPMAN ET ALENERGY CONVERTER 2 Sheets-Sheet 2 1M L'A i L- EK L L'EBEL -i NEGATIVEION ALKALI ION RECIPRICAL OF INTERATOMIC SPACING 7" e is 6o\\\ 2f {44 768 flcw wvzzk w 74 72 7Mq ATTORNEY United States Patent 3,402,074 ENERGYCONVERTER Richard A. Chapman, Richardson, Tern, and John J. Connelly,Jr., Chevy Chase, Md., asslgnors to 'lexas Instruments Incorporated,Dallas, Tex., a corporation of Delaware 79. O n l a llcatlon Mar. 221963, Ser. No. 267,1

iiiviiiea fna this applicatioii Mar. 23, 1964, Ser. No.

3 Claims. (Cl. 117 224 This application is a division of applicationSer. No. 267,179, filed Mar. 22, 1963, in the name of Richard A. Chapmanand entitled, Energy Converter. V

The present invention relates to a thermionic converter for convertingheat energy to electrical energy. Moreparticularly, it relates to athermionic converter contaming cesium vapor in the interelectrodespacing and having an improved, low thermionic work function electroncollector that is stable in the cesium vapor.

A thermionic converter consists of a hot electron emitter and a coolerelectron collector in spaced opposing relation to the emitter, and thetwo are sealed 1n a vacuum or a gas-filled enclosure. An electron 1n theemltter material can escape if it has a velocity component normal to theemitting surface with a thermal energy relative to the Fermi levelgreater than the thermionic work function of the particular emittermaterlal. As the emitter temperature is increased, the number ofelectrons which can escape the emitter is increased according to thewell-known Richardson equation. When the electrons are collected by thecollector electrode, they lose an amount of energy as heat which isequal to the work function of the collector material. If the workfunction of the emitter is greater than that of the collector, therewill be an excess potential energy available whlch, at maximum power, isequal to the difference 1n work functions. This excess potential energycan be used to provide an electrical output voltage to a load across thecollector and emitter.

The maximum power output of the converter 18 equal to the difference inthe work functions times the Richardson electron current from theemitter. Thus two ways are suggested for increasing the maximum poweroutput, which are to increase the difference between the emitter andcollector work functions or to increase the Richardson current. One wayin which the former can be accomplished is by increasing the emitterwork function, but at the same time, the disadvantage of a decrease inthe Richardson current may result. One way in which the latter can beaccomplished is by decreasing the emitter work function to increase theRichardson current, although the disadvantage of reducing the magnitudeof the difference between the emitter and collector work functionsresults. Other consequences of choosing the former is that the selectionof an emitter material of very high work function necessitates acorrespondingly high emitter operating temperature. At least twodlsadvantages result from this approach; one is that some materials ofhigh work function decompose at the temperature required to causesubstantial electron emission, and the other is that such an elevatedoperating temperature is undesirable in that a thermal source at thistemperature may not be available, special fabrication of the device isrequired to withstand the temperature involved, etc.

On the other hand, a suitable electron collector material that has avery low work function makes possible the 3,402,074 Patented Sept. 17,1968 ice use of an emitter material of intermediate work function, yetat the same time a reasonable emitter temperature can be used to producea high power output. Up until the time of this invention however, nomaterial had been found suitable for use as an electron collector thatwas characterized by a work function less than that of cesium-coatedmetals, which have a work function of about 1.6 electron-volts, with theexception of silver-oxide which had been reacted with cesium. The lattermaterial is characterized by cesium reacting with the silver-oxide toform cesium-oxide doped with silver, and this material possesses a workfunction of about 1.0 electron-volt (e.v.') at a temperature of about400 K. However, it also decomposes rapidly and has a short operatinglifetime in the continued presence of cesium vapor.

As will be explained hereinafter, thermionic converters are oftenprovided with cesium vapor in the interelectrode spacing to neutralizethe space charge barrier caused by the electron flow, and the emittercurrent is thus increased. The present invention has as its primaryobject the provision of compositions for use as the collector of athermionic converter containing cesium in the interelectrode spacing,where the collector is stable in the cesium vapor and the work functionof which is lower than heretofore obtainable. Each of the compositionsof this invention contains cesium as a major constituent thereof tocontribute to its stability in cesium vapor, Thus the converter can beoperated at a much lower temperature than previous devices and yetprovide as much or more useful power output.

Another object is to provide a thermionic diode having a stable, lowwork function collector composition where cesium gas is used in theinterelectrode spacing of the diode and one of the major constituents ofthe anode composition is cesium.

A feature of this invention is the use of cesium in conjunction with asemiconductor element to provide the improved collector hereinbeforereferred to.

Other objects, features, and advantages will become apparent from thefollowing detailed description of the invention, including preferredembodiments thereof, when taken in conjunction with the appended claimsand the attached drawing wherein like reference numerals refer to likeparts throughout the several figures, and in which:

FIGURE 1 is a simplified schematic view of a thermionic converterillustrating the principle of operation involved;

FIGURE 2 is a graphical illustration of the potential distributionbetween the emitter and collector of the converter of FIGURE 1 and showsthe relative magnitudes of the work functions of the emitter andcollector;

FIGURE 3 is a graphical illustration of the potential distributionbetween the emitter and collector of the converter of FIGURE 1 when theelectron-space charge potential barrier has been completely neutralizedby positive cesium ions interjected in the region between the emitterand collector;

FIGURE 4 is a graphical illustration of the volt-ampere characteristicsof a converter having a potential distribution as shown in FIGURE 3;

FIGURE 5 is a graphical illustration of the electron energy levels ofthe positive and negative ions of an alkalihalide compound as a functionof interionic spacing, used for the purpose of a model for the collectorcompositions of this invention;

FIGURE 6 is a side elevational view in section of one embodiment of athermionic converter; and

FIGURE 7 is a side elevational view in section of another embodiment ofa thermionic converter.

To better understand the invention, a brief discussion of a conventionalthermionic converter is given where reference is had to FIGURE 1 whichis a simplified schematic view of a thermionic converter. The convertercomprises a first electrode or emitter, the material of which is a goodelectron emitter at an elevated temperature and a spaced, opposingsecond electrode or collector for collecting the electrons from theemitter. The emitter and collector are sealed under vacuum or alow-pressure gas by a. suitable enclosure. The emitter is heated by anysuitable means to a temperature sufficientto provide sufficientelectrons with energies greater than the work function of the material,thus causing many electrons to boil off the emitter. The residualkinetic energy of the electrons causes them to travel to the surface ofthe collector, where they lose potential energy in an amount equal tothe work function of the collector material. Since the electronsoriginally had potential energy in an amount equal to the work functionof the emitter material, the maximum power available for useful workthrough a load connected externally across the emitter and collector isequal to the difference in the work functions of the emitter andcollector material times the Richardson current from the emitter.

The potential distribution of the electrons between the emitter andcollector of a vacuum sealed, thermionic converter is shown graphicallyin FIGURE 2, where the emitter is illustrated at the left. In order forthe electrons to escape the emitter, they must have velocity componentsnormal to the emitting surface which represent an energy relative to theFermi level greater in amount than the Work function 111;; of thatmaterial, which is the difference in energy between a zero kineticenergy electron just outside the surface of the emitter and the energyof an electron at the Fermi level energy of the material. In a vacuumdiode. an electron-cloud or potential barrier V hereinafter referred toas a space charge barrier is created between the emitter and collectorwhen an electron flow is established. If the electron has sufiicientkinetic energy, it will overcome this barrier and travel to thecollector. Thus the electrons in the emitter must have sufficient energyto overcome this potential barrier in addition to the work function. InFIGURE 2 the total potential required for an electron to reach thecollector is represented by V which is equal to the work function plusthe additional potential barrier due to the space charge. Thus at agiven temperature, the electron current will be reduced much below theRichardson current from the emitter, and thus the maximum power isreduced. For a more complete discussion of the thermionic diode in thisrespect, reference is had to Kaye and Welsh, Direct Conversion of Heatto Electricity, John Wiley & Sons, Inc., New York, 1960, chap. 7.

It has long been known that the space charge can be reduced orcompletely neutralized by the addition of positive ions in the spacebetween the emitter and collector. Reference is again had to chapter 7of the Kaye and Welsh publication, supra, for a discussion of thistopic. The best substance for neutralizing the space charge is cesiumgas. in which a reservoir of cesium is included within the diodeenclosure and is heated sufiiciently to convert it to a vapor. Theneutral cesium atoms in contact with the hot emitter become ionized anddrift into the diode space as positive ions, thus neutralizing the spacecharge. Cesium absorbs on the cooler collector surface in the form of afilm of ions, which as indicated earlier provide in effect a co1- lectorof low work function, viz, about 1.6 e.v. Thus the cesium serves to twopurposes, one of which is to neutralize the space charge and the otherof which is to re duce the work function of the collector. All of thisis well known in the art. A graphical illustration showing the potentialdistribution in the diode space for a completely neutralized spacecharge using cesium ions is shown in FIGURE 3, where the electrons leavethe emitter surface with a potential equal to the work function andtravel to the collector without losing or gaining potential.

In FIGURE 4 there is illustrated in graphical form the theoreticalvolt-ampere characteristics of a thermionic diode, which acts as aconstant current generator. The ordinate represents collector current,or current flowing in an external load, and the abscissa represents theoutput voltage across the external load connected across the emitter andcollector. Assuming the diode to be operating under the Richardsonsaturation current I the collector I flowing in the external load willequal I if the output voltage is less than This condition prevails whenthe load impedance is less than the internal impedance of the diode. Asthe load impedance is increased to where the output voltage is equal tomaximum efficiency will have been achieved. Application of a positiveexternal voltage in an attempt to further increase the collector currentI results in a decrease in efficiency as shown by the curve of FIGURE 4.Thus it is seen that the maximum efficiency is achieved when the outputvoltage is equal to and by increasing the magnitude of this quantity,more power output can be gained.

According to the present invention, a stable cesium compoundsemiconductor material collector is provided that is suitable for use asvery low work function collectors in a thermionic converter containingcesium vapor between the electrodes, thus maximizing the quantitywithout reducing the Richardson current. To illustrate the effectivenessof decreasing the collector work function, a converter, the emitter ofwhich is to emit 5 amps/cm. at an operating temperature of 1300 C., musthave an average effective work function of 2.42 e.v. Ideally, themaximum power density that can be delivered by the converter is equal tothe emitter Richardson current (5 amps/0111. multiplied by the quantityConventional collectors in which cesium coated refractory metals areused have a work function of about 1.6 e.v. Thus the maximum powerdensity is 5 amps/cm. times the difference in work functions (2.42e.v.-1.6 e.v.), or is equal to 4.1 watts/cm A collector material of workfunction of 1.3 e.v. (a decrease of about 18% from the higher workfunction of 1.6 e.v.) gives a power of 5.6 watts/cmP, or an increase inpower of about 36%. A further decrease of the collector work function to1.0 e.v. (a decrease of about 35% from the higher work function of 1.6e.v.) gives a power of 7.1 watts/cm or an increase in power of about73%. On the other hand the absolute percent increase in power due todecreasing the collector work function is not nearly as great when theemitter has a much higher Work function and is operated at acorrespondingly higher temperature. This follows from the fact that theratio of the difference in work functions between 1.6 e.v. and the newlower value to the quantity is relatively small. Thus the very low Workfunction collector material provided by this invention is primarilyuseful for obtaining :a high power output from a converter whose emitterwork function and its temperature of operation are relatively low.

As Will be described hereinafter, the collector is comprised of acomposition of cesium and a semiconductor material so that a low workfunction results. However, it is important to consider anotherrequirement of the Collector material, which is its stability againstevaporation and decomposition. The converter of the present invention isdesigned to operate with a given amount of cesium vapor in theemitter-collector electrode spacing so that the above-mentionedneutralizing effect can be achieved, and because cesium constitutes oneelement of the low work function collector composition. The fact thatthermionic converters are operated at elevated temperatures necessitatesthe stability requirement, and it is important that the collectorcomposition does not decompose or evaporate readily during operation.Since a certain cesium vapor pressure is present at the surface of thecollector, this will prevent or retard the evaporation of cesium fromthe collector if the collector composition tends to decompose andevaporate in its elemental states. Likewise, the vapor pressure of theelemental constituents of the composition other than cesium will belowered by the cesium pressure maintained at the collector surface, thuspreventing or retarding the evaporation of these constituents. That is,due to the cesium vapor pressure, the partial pressures just off thesurface of the collector of the constituent elements of the collectorcomposition other than cesium, whether the composition be a chemicalcompound or a mixture, will be less than the vapor pressures of the sameconstituents in a vacuum at the same temperature. In essence, the cesiumvapor pressure suppresses evaporation of the collector constituents. Ifthe collector composition is considered as a chemical compound andevaporation can only take place in the compound state, it is true, ingeneral, that the heat of vaporization of the compound is greater thanthat for any of its constituent elements. As a result of one or more ofthe foregoing reasons, the collector compositions of this inventionpossess the degree of stability required for use in a converter of thetype described.

The low work function composition of this invention is generallyclassified for purposes of explanation as semiconductor compounds,although this designation is for the purpose of what is believed to bean accurate theory for predicting these compositions for the objectshereinbefore stated, and it is to be understood that the termcomposition is used in its broadest sense including an aggregation onmixture or elements not necessarily forming a chemical compound.Moreover, specific chemical compounds mentioned hereinafter as collectorcompositions may vary in their constituents from the true stoichiometricratio expressed by the chemical formula, as where, for example, aparticular compound contains an excess amount of one of the constituentsas a donor impurity.

It has been found that collectors formed of compositions of cesium and asemiconductor material, particularly, for example, silicon and germaniumhave very low thermionic work functions and are stable in the presenceof cesium vapor at the temperature of operation of the collector of theconverter.

While we do not wish to be bound by any theories, it is thought that thebest explanation of the low thermionic work functions of the compoundcesium semiconductor-collector composition of the invention is based onthe theory of a compound with a strong ionic binding similar to thealkali-halide compounds, in which sodiumchloride is a good example. Anionic compound is one in which the metal atom, such as sodium, gives upan electron to the non-metal atom, such as the chlorine, and the atomsexist in the compound or molecule state as positive and negative ions.For a more complete description of ionic compounds,'their theory andcharacteristics, see Deker, A. J., Solid State Physics, Prentice-Hall,Inc., 1959, chapters 5, 7, and 15. A general rule-of-thumb thatindicates whether a compound is ionic in nature is when the twocomponents have greatly different electronegativities such as is usuallythe case when the two components are from two greatly different chemicalgroups and valences. Here, cesium has a valence of unity and silicon andgermanium elements, for example, each have valences of four.

Using the ionic compound as a model, the thermionic work function can betheorized in terms of the Fermi energy and the electron affinity of thecompound. Referring to FIGURE 5 there is shown a graphical illustrationof the electron energy levels of an ionic compound versus the reciprocalof the interionic spacing between the positive and negative ionsconstituting the compound. The electronic energy levels of thealkali-halide compound sodium-chloride is generally explained on thebasis of such a curve. When the ions of the compound are separated by alarge distance there is no interaction between them, and the lowestunoccupied level of the metal atom is the ionization energy I, which isthe amount of energy required to remove an electron from this level tothe free state to ionize the alkali atom. The highest occupied level ofthe halide ion is the electron afiinity x which is the amount of energyrequired to remove the extra electron from the halide ion and thusneutralize the halide atom. As the interionic distance becomes smaller,the interaction therebetween causes the electronic levels to shift asshown in the graph, and as explained in Deker, supra, pp. 369- 371.Actually, the electronic levels broaden into bands of energy as theinterionic distance becomes smaller as indicated on the graph, and atthe lattice parameter a, which is the interionic separation of the ionsin the compound state, the distance between the above electronic levels,now bands of energy rather than discrete energy levels, are determined.(Moreover, at the separation a, the widths of the bands can bedetermined.) The energy between the top of the valence band and thebottom of the conduction band is the band gap energy E and the energybetween the bottom of the conduction band and the energy of a freeelectron in a vacuum is the electron alfinity of the compound. The Fermilevel of the pure ionic compound is located somewhere between theconduction and valence bands. And the thermionic work function b whichis the amount of thermal energy required to remove an electron from thecompound, is equal to the sum of the Fermi energy E and the electronaffinity of the compound.

As was mentioned earlier, one of the primary reasons that the collectorcomposition includes cesium as a major constituent was the fact thatcesium vapor is used in the interelectrode spacing of the converter toneutralize the potential barrier to current flow, and that cesiumsemiconductor compositions are found to be stable in a cesium vapor.Thus by providing a collector material or compound, the metalconstituent of which is the same element as that which is used toperform the above-noted neutralization effect, the vapor can be used toperform yet another function, viz. excess metal is incorporated into themate-rial to lower the thermionic work function. The amount of excessmetal incorporated in the collector is governed by the temperature ofthe collector and the pressure of cesium vapor surrounding it.

It is believed that the thermionic work function of each of the cesiumcomposition or semiconductor compounds of this invention is predicted inat least some degree by the model and theory alluded to above inconnection with alkali-halides. Moreover, it is believed that the betterexplanation of these compositions is based on the theory that each is acompound, where the chemical formulae are in the silicon and germaniumexamples, Cs Si and Cs Ge. The use of the compound semiconductorcomposition as a collector in the presence of cesium vapor causes anexcess amount of cesium to go into the composition. The ratio of theconstituents of the compound therefore varies from the truestoichiometric ratio of the compound, and the excess cesium metal actsas a donor impurity which decreases the thermionic work function.

The preferred method of fabricating the collector which is to be dopedwith cesium metal is to deposit by evaporation, sputtering,electroplating, or any other suitable method, onto a chemically stable(high temperature) electrical conductor, such as nickel, a semiconductorelement, for example germanium or silicon, in thickness of from severalhundred to several thousand angstrom units. The deposited surface isthen incorporated in a converter as the collector by forming a vacuumtight enclosure or being situated therein. The collector is then heated,say from C. to 200 C., for example, to drive off any gas as theenclosure is evacuated. Subsequently, cesium vapor is introduced in theenclosure, and the converter is then operated by heating the emitter toits operating temperature, for example, in excess of 1200 C. During theinitial period of the operation, the cesium reacts with thesemiconductor element deposited on the collector surface to form thecesium compound semiconductor composition. A time of about one hour isusually sufficient to substantially complete the reaction between thesemiconductor collector element and the cesium. Because of the cesiumvapor present at the surface of the collector, the collector compositionis essentially saturated with cesium in that an excess of cesium isessentially saturated with cesium in that an excess of cesium isincorporated in the composition or compound as a donor impurity.Representative of operating conditions for any one of theaboveenumerated collector compositions are an emitter temperature inexcess of 1200 C. to 1300 C., and a collector temperature of less than300 C., the latter being controlled by its physical spacing from theemitter and/ or by an independent heat source or heat sink. A cesiumreservoir is incorporated in the converter, as will be seen below withreferences to FIGURES and 6, to supply the cesium vapor in theinterelectrode spacing, and the temperature of this reservoir is usuallymaintained at a temperature slighlty less than that of the collector. Inthis way the cesium pressure in the converter is, to a large extent,governed by the coldest part of the converter, which is the cesiumreservoir. Representative of the cesium vapor pressure for the collectorcompositions aboveenumerated is from about 1O mm. of Hg to about 2.0 mm.of Hg, where the corresponding cesium reservoir temperatures are fromabout 25 C. to about 300 C. All of the above representative times,temperatures, pressures, etc., are not critical and are given forillustrative purposes only. Within wide limitations the work functionsof the various cesium collector compositions have been found to be lessthan that of pure cesium and to be stable against evaporation anddecomposition. Representative of the range of work functions for thecompositions is from about 1.1 e.v. to about 1.7 e.v. For example, thework function of Cs Si ranges between about 1.5 e.v. about 1.7 e.v.

In general, the lowest thermionic work function compositions are thosecontaining a large amount of cesium donor impurity concentration.However, it is not to be understood that no limit exists as to theamount of excess cesium dopant used. On the contrary, the amount ofexcess cesium should not be so great that it no longer acts as animpurity and tends to raise the work function back to that of purecesium. Such changes and excesses can easily be detected by measuringthe work function by the commonly used contact-potential method.

Two examples of thermionic converters are shown in FIGURES 6 and 7 wherethe first embodiment has planar emitter and'collector surfaces inparallel, opposing relation, and the second embodiment has cylindricalemitter and collector surfaces, with the collector surface surroundingthe emitter. In particular, there is shown in FIGURE 6 a converterdesigned to operate at a relatively low emitter temperature, say 1300C., to provide a power output of 5 watts at maximum efiiciency. Toobtain the necessary emitter current at the relatively low temperature,a material whose work function is from about 2.0-2.5 e.v. must be used.This intermediate work function emitter can either be a refractory metalpartially covered with cesium or a dispenser cathode which comprises ahigh temperature metal, such as tungsten or tantalum, which has beenimpregnated by a lower work function element, such as barium orstrontium. The dispenser cathode is well known and will not beelaborated on here. The converter is comprised of a first electrode 28of one of the materials named above wtih a planar surface 29, and asecond electrode 20 having a planar surface 24 in spaced, opposingrelation to the emitter surface and onto which has been deposited one ofthe collector compositions of this invention. The first or emitterelectrode has the active emitter portion extended toward the collector,.as shown, so that the emitter and collector surfaces are in closeproximity. The second or collector electrode is made with suitable metalsubstrate, such as nickel, and has an aperture 27 therethrough as shown,into which is sealed a reservoir 36 for containing a small amount ofcesium 40. The electrodes are supported in their respective positions bycylindrical metallic sealing flanges 31 and 33 sealed to the peripheriesof the first and second electrodes, respectively, and the outer edges ofthe flanges are sealed to a cylindrical ceramic member 32. The flangesact as bellows in the sense that they are able to expand with thermalstresses to allow for the large temperature gradient between the emitterand collector. The flanges have small cross-sectional areas to limit theheat flow by conduction from emitter to collector to a minimum, and alsoserve as an enclosure for the interelectrode spacing. A load isconnected across the emitter and collector electrode by leads 44 and 44,respectively.

The converter is operated by heating the emitter by any suitable means,such as by direct thermal contact with a hot reservoir, or by a solarcollector, as examples, and the collector is heated to about C. to about300 C,. normally by radiation from the emitter. Since the coldestsurface within the enclosure determines, to a large extent, the cesiumvapor pressure in the space between the emitter and collector, thepressure can be easily regulated by controlling the temperature of thecesium reservoir. The cesium reservoir is heated to a slightly elevatedtemperature, say from 25 C., to about 300 C., to establish the desiredamount of cesium vapor pressure within the enclosure, such as by heatconduction from the collector. Under normal operating conditions thecollector is operated at a slightly higher temperature than the cesiumreservoir to prevent an undue amount of cesium from condensing on thecollector surface.

A cylindrical configuration of a converter is shown in FIGURE 7 that isprimarily adapted to a nuclear heat source. A cylindrical ceramic member64 supports both the emitter and collector electrodes 60 and 62,respectively. The emitter electrode is feathered or thinned out at itsupper end 66 and sealed to the inner surface of the ceramic member, andthus when the emitter electrode is heated, the thinned portion allowsflexibility in response to thermal stresses. The emitter electrodeconsists of a cylindrical can closed at its bottom end and into theinterior 70 of which can be inserted a heat source, such as a nuclearfuel element. The collector electrode comprises a cylindrical can havingan aperture through its lower end and into which is sealed a reservoir72 for containing a small amount of cesium 74. This electrode, like thatof FIGURE 6, has .an extended portion onto the surface 68 of which isdeposited one of the compositions of the invention. The materialscomprising the electrodes and the operating conditions for the converterare essentially the same as those for the converter of FIG- URE 6.

The foregoing descriptions of converters with reference to FIGURES 6 and7 are for illustrative purposes only and .are in no way to be construedin .a limiting sense. Rather, the invention constitutes a new collectorcomposition for use in combination with a thermionic converter and is tobe limited only as defined in the appended claims.

What is claimed is:

1. A collector for a thermionic converter of the type having an emitterand a collector with cesium vapor disposed in the closed space betweenthe emitter and the collector, said collector consisting essentially ofan electrically conductive substrate chemically stable at hightemperatures coated with a composition of cesium and an element selectedfrom the group consisting of silicon and germanium and containing adonor impurity from Group I-A of the Periodic Table of Elements in anamount sufficient to lower the thermionic work function of saidcomposition. I

9 2. A collector according to claim 1 wherein said substrate is nickel.

3. A collector according to claim 1 wherein said donor impurity iscesium.

References Cited UNITED STATES PATENTS 10 3,002,116 9/1961 Fisher 310-43,121,048 2/1964 Haas 3104 FOREIGN PATENTS 854,036 11/1960 GreatBritain.

OSCAR R. VERTIZ, Primary Examiner.

H. S. MILLER, Assistant Examiner.

1. A COLLECTOR FOR A THERMIONIC CONVERTER OF THE TYPE HAVING AN EMITTERAND A COLLECTOR WITH CESIUM VAPOR DISPOSED IN THE CLOSED SPACE BETWEENTHE EMITTER AND THE COLLECTOR, SAID COLLECTOR CONSISTING ESSENTIALLY OFAN ELECTRICALLY CONDUCTIVE SUBSTRATE CHEMICALLY STABLE AT HIGHTEMPERATURES COATED WITH A COMPOSITION OF CESIUM AND AN ELEMENT SELECTEDFROM THE GROUP CONSISTING OF SILICON AND GERMANIUM AND CONTAINING ADONOR IMPURITY FROM GROUP I-A OF THE PERIODIC TABLE OF ELEMENTS IN ANAMOUNT SUFFICIENT TO LOWER THE THERMIONIC WORK FUNCTION OF SAIDCOMPOSITION.